Support `$startIndex` param on `Pos` method

[nedmas]

Description

The Pos method used in SilverStripe template loops supports adjusting the starting number of the position index (see silverstripe-framework/src/View/SSViewer_BasicIteratorSupport.php). It would be good to add feature parity for those wishing to use zero based indexes ;)

Additional context or points of discussion

No response

Validations

• [X] You intend to implement the feature yourself
• [X] You have read the contributing guide
• [X] You strongly believe this feature should be in core, rather than being its own community module
• [X] You have checked for existing issues or pull requests related to this feature (and didn't find any)

PRs

• https://github.com/silverstripe/silverstripe-elemental/pull/1225

[GuySartorelli]

Hiya. As mentioned in your original PR, this can only be done in a major release. Feel free to open a new pull request targeting the 6 branch.